Then, having already realised she is between the rock of a "no deal" and the hard place of an unattainable free trade deal, she can devote her attention to crafting a "not the EEA" agreement that covers all the same bases but qualifies as something new and different.

Brexit deadline a specific factor in snap poll move

Under the Fixed Term Parliaments Act, that's when the Tories would be starting to prepare for a general election the following year, with what one cabinet minister described as certain "political needs".

In other words, the government would be exposed to hardball from the EU because ministers would be desperate to avoid accepting anything that would be politically unpopular, or hold the Brexit process up, at the start of a crucial election cycle.

As one insider put it: "We'd be vulnerable to the rest of the EU in 2019 because they know we'd have to move fast."

Ministers say that's the central reason for May's change of heart because "if there was an election in three years, we'd be up against the clock".

By holding the election now, Theresa May hopes she gains a fresh start on the political clock on Brexit, even though the Article 50 process still has a deadline of only two years.

This gives what was described as "flexibility over the logistics of Brexit… we don't have to pretend we can do it all in two years".

'Window' of opportunity

That's not to say for a second this means departure from the EU will be held up. The Article 50 process will proceed, the government says, exactly as planned.

But, if the Tories win, an early election may have bought ministers some valuable breathing space to work out what the UK really looks like outside the EU.

They'll have, in theory, three years after exit for things to settle down before the public get another chance to have their say.